Mr. Cohen comes to the project from George Mason University, where he directs the Roy Rosenzweig Center for History and New Media. In the announcement, John Palfrey, president of the DPLA’s Board of Directors, praised Mr. Cohen’s contributions to libraries and digital scholarship.

“He has led major open-source development projects, helped to digitize important works of culture, supported teachers and students in accessing fantastic digital materials, and written about the importance of libraries, archives, and museums in a digital age,” Mr. Palfrey wrote. (Mr. Cohen was named one of The Chronicle’s top tech innovators last year.)

Mr. Cohen will move to Boston this year to assume his new duties. In an interview, he called the chance to help build the DPLA “one of those once-in-a-lifetime opportunities.” The idea of “a large-scale online library is something truly special for the 21st century,” he said.

He sketched out a vision of the DPLA as both a gatherer of information and a gateway to it. It will be “an important nationwide collaboration of state and regional digital libraries who will bring together all the local content and bring it upstream to this giant ocean that will be the DPLA,” he said. “The DPLA, in turn, will redirect the general public and scholars and teachers” to digital collections and cultural resources across the country.

“The idea that we can bring all this content to Americans and people all across the world is tremendously compelling,” Mr. Cohen said.

A prototype DPLA portal will make its debut in Boston next month, to give people a sense of what “the public-facing part of this” will ultimately look like, according to the new executive director. Behind the scenes, an open interface will allow software developers to build on the DPLA platform.

Mr. Cohen said he intended to make advocating for greater public access to information a key part of his new job. “Right now we can see many of the ways in which cultural materials are being locked up or attached with DRM [digital rights management],” he said. “We really need a national organization to say, ‘We need a strong public option.’”

What worries him most at the moment, he said, is that ordinary people, whom the DPLA is meant to serve, don’t yet know anything about it. Inside this “strange bubble world” of libraries and digital humanities, the DPLA has long been a subject of lively discussion. But “99 percent of Americans have not heard of this before,” he said. “My concern right now is one about visibility and making sure that the general public understands what the DPLA is doing and that it will not be a replacement for public libraries.”

In Mr. Cohen’s view, the DPLA will not supersede those libraries but build on their mission of putting information in the hands of the public. “One of the great things about libraries is that they are open for all,” Mr. Cohen said. “We want to strengthen what public libraries already do in a digital age.”